Robert Mueller, the former FBI director and current special prosecutor in the Russia case, once was hauled before the nation’s secret intelligence court to address a large number of instances in which the FBI cheated on sensitive surveillance warrants, according to evidence gathered by congressional investigators.

But thanks to recent testimony from a former FBI lawyer, we now have a rare window into documented abuses of Foreign Intelligence Surveillance Act (FISA) warrants and how the courts handled the matter.

The episode is taking on new significance as Mueller moves into the final stages of his Russia probe while evidence mounts that the FBI work preceding his appointment as special prosecutor may have involved improprieties in the securing of a FISA warrant to spy on Donald Trump’s campaign in the final weeks of the 2016 campaign.